The present invention relates to exposure technology and, more particularly, to a technology which is effective if applied to the photolithography process of a semiconductor integrated circuit device.
As the high integration of the semiconductor integrated circuit advances so that the circuit elements and the wiring design rules come to the sub-micron order, the photolithography process for transferring a circuit pattern on a mask onto a semiconductor wafer by making use of a beam of g- or i-line is troubled by a serious problem of a reduction in the precision of the circuit pattern to be transferred onto the wafer. In case, for example, a circuit pattern formed of transparent regions P.sub.1 and P.sub.2 and a shielding region N over a mask 20, as shown at (a) in FIG. 1J, is to be transferred onto the wafer, the phases of the light beams L just after having passed through the paired transparent regions P.sub.1 and P.sub.2 interposing the shielding region N are in phase with each other, as shown at (b) in the same Figure. As a result, the two light beams interfere with each other at that portion on the wafer, which might otherwise be intrinsically the shielding region, so that they are intensified (as shown at (c) in the same Figure). As a result, the contrast of the projected image on the wafer drops with a reduced focal depth, as shown at (d) in the same Figure, so that the pattern transfer precision is seriously degraded.
As means for solving this problem, there has been proposed a phase shift technology for preventing the drop of the contrast of the projected image by changing the phases of the light beams that transmit through the mask. In Japanese Patent Publication No. 59296/1987, for example, there is disclosed a phase shifting technology, in which one of the paired transparent regions across the shielding region is formed with a transparent film to establish a phase difference between the light beams having passed through the two transparent regions at the time of exposure so that the interfering beams may be weakened at that portion on the wafer, which might otherwise be the shielding region. When a circuit pattern formed on a mask 21, as shown at (a) in FIG. 1K, is to be transferred onto a wafer, either of the paired transparent regions P.sub.1 and P.sub.2 interposing the shielding region N is formed with a transparent film 22 having a predetermined refractive index. By adjusting the thickness of the transparent film 22, moreover, the individual light beams having passed through the transparent regions P.sub.1 and P.sub.2 go out of phase by 180 degrees, as shown at (b) in the same Figure, so that they interfere with each other in the shielding region N on the wafer and are weakened (as shown at (c) in the same Figure). As a result, the contrast of the projected image on the wafer is improved, as shown at (d) in the same Figure, to improve the resolution and the focal depth and accordingly the transfer precision of the circuit pattern formed on the mask 21.
In Japanese Patent Laid-Open No. 67514/1987, on the other hand, there is disclosed a phase shift technology in which a phase difference is established between a light beam having passed through a transparent region and a light beam having passed through a fine opening pattern, by removing the shielding region of a mask partially to form the opening pattern and by forming a transparent film in either the opening pattern or the transparent region existing in the vicinity of the opening pattern, so that the light beam having passed through the transparent region may be prevented from having its amplitude distribution expanded transversely.
The phase shifting method, by which one mask is formed thereon with an ordinary pattern (or main pattern) and a shifter pattern (or accompanying or complementary pattern) for giving a phase opposed to that of the former, will be called hereinafter the "on-mask phase shifting method" and will be called the "on-mask phase inversion shifting method" especially in case the phase shift is (2n+1).rho. (wherein n: an integer).
In Japanese Patent Laid-Open No. 109228/1985, moreover, there is disclosed a method, in which two masks are simultaneously illuminated to improve the throughput of a projecting exposure so that the portions of one wafer corresponding to different chips may be simultaneously exposed. In Japanese Patent Laid-Open No. 107835/1985, on the other hand, there is disclosed a technology, in which two masks having an identical pattern can be exposed without any trouble even if one of them is defective, by dividing one exposing line into two halves to illuminate the identical portion of the two masks and by composing them to expose the wafer.
However, these two disclosures are not effective in the least for improving the resolution although they are effective for preventing the defect on the mask pattern from being transferred onto the wafer or for improving the throughput.